A particle lies on a line perpendicular to a thin circular

Question

Question: A particle lies on a line perpendicular to a thin circular ring and through its center. The radius of the ring is a, and the distance from the point to the center of the ring is y. For a positive constant K, the gravitational force exerted by the ring on the particle is given by

F =        Ky      
     (a² + y²)^3/2

If y is increasing, how is F changing with respect to time, t, when

a) y = 0

b) y = a/√2

c) y = 2a